Sintered ferrite magnets are used in various applications such as motors, power generators, speakers, etc. Known as typical sintered ferrite magnets are Sr ferrite (SrFe12O19) and Ba ferrite (BaFe12O19) each having a hexagonal M-type magnetoplumbite structure. These sintered ferrite magnets are produced from starting materials comprising iron oxide, carbonates of strontium (Sr) or barium (Ba), etc. by a powder metallurgy method at a relatively low cost.
Recently, to provide smaller, lighter-weight, higher-performance electronic parts for automobiles, parts for electric appliances, etc. for environmental advantages, sintered ferrite magnets are required to have higher performance. Particularly desired for motors for automobiles are sintered ferrite magnets having, in addition to high residual magnetic flux density Br, such high coercivity HcJ as to avoid demagnetization by a demagnetizing field generated when thinned.
To provide sintered ferrite magnets with improved magnetic properties, proposals have been made to improve coercivity HcJ and a residual magnetic flux density Br by substituting part of Sr with a rare earth element such as La, etc., and substituting part of Fe with Co in the above Sr ferrite (for example, JP 10-149910 A, and JP 11-154604 A).
The Sr ferrite with part of Sr substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Sr—La—Co ferrite”), which is described in JP 10-149910 A and JP 11-154604 A, has excellent magnetic properties, finding various applications in place of conventional Sr or Ba ferrite. However, further improvement in magnetic properties is desired.
Also known for the sintered ferrite magnets together with the above Sr or Ba ferrite is Ca ferrite. It is known that the Ca ferrite has a stable structure represented by the formula of CaO—Fe2O3 or CaO—2Fe2O3, forming hexagonal ferrite by the addition of La. However, it has magnetic properties on substantially the same levels as those of conventional Ba ferrite, not sufficiently high.
To improve a residual magnetic flux density Br and coercivity HcJ as well as the temperature characteristics of coercivity HcJ, Japanese Patent 3181559 discloses Ca ferrite with part of Ca substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Ca—La—Co ferrite”), which has an anisotropic magnetic field HA of 20 kOe or more, higher by 10% or more than that of Sr ferrite.
However, the Ca—La—Co ferrite described in Japanese Patent 3181559 has substantially the same Br and HcJ as those of Sr—La—Co ferrite, despite a higher anisotropic magnetic field HA than that of the Sr—La—Co ferrite. In addition, having an extremely poor squareness ratio, it does not meet high coercivity and a high squareness ratio simultaneously, failing to be used for various applications such as motors, etc.
With respect to the Sr—La—Co ferrite proposed by JP 10-149910 A and JP 11-154604 A, JP 2007-123511 A proposes a method for improving HcJ while maintaining Br. In general, a sintered ferrite magnet is produced by the steps of (1) mixing iron oxide with carbonate of Sr or Ba, etc. as starting materials, (2) calcining a starting material mixture for ferritization to obtain a calcined body, (3) pulverizing the calcined body to powder, (4) molding the powder to a green body, and (5) sintering the green body to obtain a sintered body. JP 2007-123511 A discloses a pulverization step comprising a first fine pulverization step, a powder heat-treating step and a second fine pulverization step, by which the percentage of crystals as large as 1.1 μm or less is made 95% or more, improving HcJ.
Though HcJ is improved by the method of JP 2007-123511 A, cost increase is unavoidable due to increase in the number of production steps. Particularly, Sr—La—Co ferrite containing expensive elements such as Co, La, etc. as indispensable components suffers a higher material cost than conventional Sr ferrite. Higher production cost deprives sintered ferrite magnets of the most important economic advantages, failing to meet cost requirements in the market.
Various measures for improving performance have been conducted on the Ca—La—Co ferrite proposed by Japanese Patent 3181559. For example, JP 2006-104050 A proposes Ca—La—Co ferrite containing La and Co at predetermined percentages with the molar ratios of elements and the value of n optimized. WO 2007/060757 proposes Ca—La—Co ferrite with part of Ca substituted by La and Ba. WO 2007/077811 proposes Ca—La—Co ferrite with part of Ca substituted by La and Sr. However, the Ca—La—Co ferrites of JP 2006-104050 A, WO 2007/060757 and WO 2007/077811 fail to provide sintered ferrite magnets having high HcJ satisfying demands in the market.
JP 2008-137879 A and WO 2008/105449 disclose a technology for improving HcJ by using the pulverization step described in JP 2007-123511 A in the production of Ca—La—Co ferrite. Though the Ca—La—Co ferrites described in JP 2008-137879 A and WO 2008/105449 have improved HcJ, they doubly suffer increase in a material cost and a production cost as in JP 2007-123511 A, failing to meet cost requirements in the market.
Thus, sintered ferrite magnets meeting recent requirements of magnetic properties and costs have not been proposed yet.
It is known to add SiO2, CaCO3, etc. as sintering aids to sintered ferrite magnets, to change balance between Br and HcJ, which are in a trade-off relation in the sintered ferrite magnets. To obtain high Br, it is effective to reduce the amounts of sintering aids acting as non-magnetic components in a range having liquid phase components necessary for sintering, or to increase the amount of CaCO3 relative to that of SiO2. However, such measures make it difficult to maintain fine sintered structures, resulting in low HcJ. On the other hand, to obtain high HcJ, it is effective to increase the amounts of sintering aids, or to increase the amount of SiO2 relative to that of CaCO3. However, such measures increase the amounts of non-magnetic components or lower sinterability, inevitably resulting in decrease in Br and a squareness ratio Hk/HcJ, wherein Hk was the value of H at J of 0.95Br on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant.
In conventional sintered ferrite magnets, particularly in Sr—La—Co ferrite and Ca—La—Co ferrite proposed recently, the amounts of sintering aids such as SiO2, CaCO3, etc. are generally as small as possible to maintain high Br. For example, JP 2006-104050 A describes that 0.3-1.5% by mass (calculated as CaO) of CaCO3 and 0.2-1.0% by mass of SiO2 are added preferably at the time of pulverizing a calcined body. WO 2007/060757 describes that 0.2-1.5% by mass (0.112-0.84% by mass when calculated as CaO) of CaCO3, and 0.1-1.5% by mass of SiO2 are added preferably at the time of pulverizing a calcined body. JP 2008-137879 A describes that 1.35% or less by mass of SiO2 is preferably added to a sintering material.
However, JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A describe in Examples only Ca—La—Co ferrite magnets, in which the amounts of both SiO2 and CaCO3 (calculated as CaO) are 0.9% or less by mass. Because emphasis is placed on the improvement of the Br of these ferrite magnets, the addition of SiO2 and CaCO3 in amounts of more than 0.9% by mass is not contemplated, providing no information about the magnetic properties (Br, HcJ and Hk/HcJ) of such Ca—La—Co ferrite magnets.